A study published in *Nature* (2014) reveals that folate-dependent NADPH production is a significant source of reducing power in proliferating cells. The research uses deuterium and carbon isotope tracers combined with mathematical modeling to track NADPH fluxes. The oxidative pentose phosphate pathway (oxPPP) is the primary source of cytosolic NADPH, but serine-driven one-carbon metabolism also contributes significantly. This pathway involves the oxidation of methylene tetrahydrofolate to 10-formyl-tetrahydrofolate, which is then reduced to NADPH. Mitochondrial one-carbon metabolism also contributes to NADPH production by fully oxidizing 10-formyl-tetrahydrofolate. Knockdown of methylenetetrahydrofolate dehydrogenase (MTHFD) genes reduces cellular NADPH/NADP+ and GSH/GSSG ratios and increases cell sensitivity to oxidative stress, confirming the functional significance of folate metabolism in NADPH production. The study also shows that the oxPPP contributes 30-50% of overall NADP+ reduction. Using a genome-scale metabolic model, the researchers found that both the oxPPP and malic enzyme contribute approximately 30% of NADPH production. However, one-carbon metabolism mediated by tetrahydrofolate (THF) accounts for about 40% of NADPH production. The main folate-dependent NADPH-producing pathway involves the transfer of a one-carbon unit from serine to THF, followed by oxidation of the resulting product by MTHFD to form formyl-THF with concomitant NADPH production. This pathway was confirmed by observing labeling of NADP+ and NADPH after feeding cells 2,3,3-2H-serine. The study also highlights the importance of mitochondrial folate metabolism in NADPH production. Mitochondrial folate-dependent enzymes, especially MTHFD2, are overexpressed in human cancers. The glycine cleavage system, which is involved in one-carbon metabolism, contributes to mitochondrial NADPH production. The study found that the complete oxidation of mitochondrial methylene-THF to CO2 is essential for redox homeostasis. The results suggest that mitochondrial folate metabolism has two functions: glycine detoxification and NADPH production. The study also shows that NADPH is primarily used for biosynthesis rather than redox defense. In proliferating cells, most cytosolic NADPH is devoted to biosynthesis, not redox defense. The study found that the overall demand for NADPH for biosynthesis is greater than 80% of total cytosolic NADPH production, with a majority of this NADPH consumed by fatty acid synthesis. The study also shows that the production of NADPHA study published in *Nature* (2014) reveals that folate-dependent NADPH production is a significant source of reducing power in proliferating cells. The research uses deuterium and carbon isotope tracers combined with mathematical modeling to track NADPH fluxes. The oxidative pentose phosphate pathway (oxPPP) is the primary source of cytosolic NADPH, but serine-driven one-carbon metabolism also contributes significantly. This pathway involves the oxidation of methylene tetrahydrofolate to 10-formyl-tetrahydrofolate, which is then reduced to NADPH. Mitochondrial one-carbon metabolism also contributes to NADPH production by fully oxidizing 10-formyl-tetrahydrofolate. Knockdown of methylenetetrahydrofolate dehydrogenase (MTHFD) genes reduces cellular NADPH/NADP+ and GSH/GSSG ratios and increases cell sensitivity to oxidative stress, confirming the functional significance of folate metabolism in NADPH production. The study also shows that the oxPPP contributes 30-50% of overall NADP+ reduction. Using a genome-scale metabolic model, the researchers found that both the oxPPP and malic enzyme contribute approximately 30% of NADPH production. However, one-carbon metabolism mediated by tetrahydrofolate (THF) accounts for about 40% of NADPH production. The main folate-dependent NADPH-producing pathway involves the transfer of a one-carbon unit from serine to THF, followed by oxidation of the resulting product by MTHFD to form formyl-THF with concomitant NADPH production. This pathway was confirmed by observing labeling of NADP+ and NADPH after feeding cells 2,3,3-2H-serine. The study also highlights the importance of mitochondrial folate metabolism in NADPH production. Mitochondrial folate-dependent enzymes, especially MTHFD2, are overexpressed in human cancers. The glycine cleavage system, which is involved in one-carbon metabolism, contributes to mitochondrial NADPH production. The study found that the complete oxidation of mitochondrial methylene-THF to CO2 is essential for redox homeostasis. The results suggest that mitochondrial folate metabolism has two functions: glycine detoxification and NADPH production. The study also shows that NADPH is primarily used for biosynthesis rather than redox defense. In proliferating cells, most cytosolic NADPH is devoted to biosynthesis, not redox defense. The study found that the overall demand for NADPH for biosynthesis is greater than 80% of total cytosolic NADPH production, with a majority of this NADPH consumed by fatty acid synthesis. The study also shows that the production of NADPH